1. Field of the Invention
The present invention generally relates to steering mechanisms for long vehicles, including, among others, trucks, busses, stretch limos, and semi-tractors and trailers. More specifically, the present invention pertains to a rear wheel steering system for long vehicles, enabling the vehicles to safely and easily clear an inside curb when making sharp turns on narrow city streets.
2. Description of the Related Art
Presently, long vehicle turns in tight situations can be quite dangerous. For example, when turning from one city street to another, the driver of a large trailer rig must begin the turn in the second lane over from the curb, thus taking up both lanes of traffic, in an effort to prevent the rear tandem wheels of the rig from running over the curb and perhaps up on the sidewalk. Similarly, long vehicle turns across four lane intersections can be dangerous. If the trailer is crossing a four lane road, it may take two lanes into the intersection, and take two lanes as it turns.
If the semi is pulling two trailers, the above problem is compounded. In such a situation, the semi turns into the intersection, the first trailer cuts across the curb, and the second trailer crosses on the sidewalk. Long trucks, busses and stretch limos, like the trailers, also must take more than their share of the road when turning sharp corners.
There have been several attempts to overcome the above disadvantages. Some long ladder fire trucks and ladder trailers have a seat and steering wheel over the rear wheels, and a fireman steers the back wheels so as to miss the curb. Steering the back wheels of a vehicle is old art. An early automobile had both front and back wheels that were steerable. Yet, directly steering the rear wheels of a vehicle has its share of problems as well, including the need of an additional driver, just to name one.
It would be beneficial to provide a rear wheel steering system that a single driver can operate by the push of a button. Such a system would improve truck hauling safety. In view of the prior art it can be seen that there is a need for a modified rear wheel steering system for long vehicles. It is to the provision of such a system that the present invention is primarily directed.
The present invention is a rear wheel steering system that enables a long vehicle to make tighter turns than are possible with conventional steering assemblies. The present rear wheel steering system requires only minimal intervention by the driver, and allows the trailerrig, truck or bus to make sharp turns without crossing into the other lane, yet providing for the rear wheels to miss the curb.
The present rear wheel steering system comprises a control system and a gyrocompass assembly housed on a pivotal rear wheel unit. The control system is capable of automating the steering process of the rear wheels, while the gyrocompass assembly is capable of defining the path for the rear wheels to follows.
The description of the present invention will include specific references to a tractor-trailer vehicle type, but it will be understood that the steering system is useful in all manner of vehicle types that incorporate rear wheel assemblies. The term xe2x80x9clengthwise centerlinexe2x80x9d will refer to the centerline of the tractor-trailer running from the front end to the back end of the cab. Thus, if the cab and trailer are in alignment, for example, in normal highway driving conditions, the lengthwise centerline would in fact extend from the front of the cab to the end of the trailer. The term xe2x80x9crear wheel unit lengthwise centerlinexe2x80x9d may, or may not, lie on the lengthwise centerline of the cab. During normal hauling, the centerlines would lie on atop the other. But when the rear wheel unit pivots relative to the length of the trailer, the rear wheel unit lengthwise centerline will similarly angle away from the trailer""s lengthwise centerline.
The present steering system has a xe2x80x9cnormal drivingxe2x80x9d state and an xe2x80x9cautomated turnxe2x80x9d state. In the normal driving state, the rear wheels are locked in alignment with the vehicle""s centerline, like any standard trailer. The normal driving state is the typical highway hauling use of the vehicle. As the long vehicle approaches a sharp turn, the driver of the vehicle activates the control system of the rear wheel steering system from the normal driving state to the automated turn state, wherein the rear wheels are then steerable.
The pivotal rear wheel unit enables the rear wheels to turn through a turning angle about the lengthwise centerline of the vehicle. In city driving applications, the wheels may be powered by fluid cylinders, electric motors or other standard means.
While the vehicle is on the straight highway, the system is in the normal driving state, and the present system is deactivated. The driver can then activate the system via the control system when entering a turn. Upon activation of the present system, the gyrocompass assembly that is mounted on the rear wheel unit begins to operate, and the rear wheel unit is unlocked from the conventional highway hauling non-pivotable rear wheel unit orientation.
The gyrocompass assembly incorporates a gyrocompass, which is a two degree of freedom stable platform having a gyroscope (or xe2x80x9cgyroxe2x80x9d) supported in gimbal rings. The gyroscope is rotatable about a vertical axis. The centerline of the gyro is normally maintained in alignment with the trailer""s lengthwise centerline by biasing elements of the gyrocompass assembly, but once energized, the gyro can resist turning with a force greater than that of the biasing elements. The biasing elements can be springs.
When the semi begins to turn, the trailer also begins to turn, and the rear wheels of the trailer are subjected to a turning force. Yet, the gyro still points as it did when it started (aligned in the lengthwise centerline of the vehicle), so when the trailer turns the trailer frame presses against a gyro frame of the gyrocompass assembly. Upon such contact, a switch to a solenoid valve closes, which operates a turning mechanism of the gyrocompass assembly. The turning mechanism can comprise at least one cylinder. For example, when the switch is activated, one of two hydraulic cylinders begins to extend. As the cylinder is attached at one end to the trailer and the other end to the rear wheel unit, when the cylinder begins to extend, it pushes (or pulls) the unit about the pivot, to relieve the contact between the trailer frame and the gyro frame. This maintains the rear tandem wheels in alignment now with the gyro""s centerline, as opposed to the trailer""s centerline.
When the semi enters the intersection and turns down the cross street, the front of the trailer turns at an angle to the semi. However, the rear wheels of the trailer do not turn with the trailer but roll straight down the road guided by the gyrocompass assembly. As the semi continues down the cross street, the turning angle between the lengthwise centerline of the trailer and the centerline of the rear wheels becomes increasing larger, until a preset turning angle is reached, opening a switch of the control system. The rear wheels are then brought back into alignment with the trailer""s centerline.
The rear wheels of very long trailers are provided with a turning angle of approximately 45 degrees. This means that about one-half of the trailer has moved into the cross street before the rear of the trailer starts to turn. The wheels thus easily miss the curb, but the body of the trailer might overlap, unless the curb is rounded off.
The turning angle that the wheels can pivot on long, self powered equipment like trucks, busses, and automobiles might be limited by the drive shaft angle, but the turn can be shortened by having a maximum turning angle of approximately 30 degrees. The turn of the rear wheels does not start until the maximum turning angle is reached, and then they would need to turn only (90xe2x88x9230=60) degrees more. A standard front wheel drive set of wheels would allow a greater angle and still take a standard drive shaft.
When the trailer or truck is fully in the cross street and the wheels have realigned with the vehicle""s centerline, they are locked in position for highway hauling.
The control system can be adjusted to allow the driver some control over the angle the wheels can turn and the time to return. For example, the rear trailer wheels can be guided straight ahead to the intersection by the gyrocompass assembly, as it is when it is controlled automatically. As the trailer turns, it presses a switch against the gyroscope frame, causing the steering cylinder to keep the wheels aligned with the gyroscope, and the gyroscope remains aligned with the position of the trailer when the gyro was energized. The rear wheels of the trailer continue to roll straight towards the intersection. When the trailer has turned through an angle to the rear wheels of, for example, 45 degrees, it hits a stop on the tandem wheel unit and can not further turn. The trailer""s wheels remain at this angle, so that the rear of the trailer moves the rest of the way into the intersection at the angle of the wheels. As the rig comes into alignment, the driver switches the controls back to the normal driving position.
For normal sized trailers, trucks, busses and the like, the rear wheels may be returned to the centerline automatically. But for very long vehicles, or two trailers in line, it would be best to let the trailer continue into the intersection at the maximum turning angle of the wheels.
Before the steering system of the present invention, there were few very long trucks, busses or fire engines, but once it is known that such vehicles can make sharp turns if they have the present rear wheel steering system, they may become more widespread.